The present invention relates to connectivity between a user equipment (UE) aboard a high-speed train and network nodes of a cellular communications system, and more particularly to technology that allows handover between different cells to be performed by the UE travelling on a train at high speed.
Depending on relative movement of a UE to a transmitting site, the received signal may display a significant Doppler shift. The Doppler shift forces the UE to increase its demodulation frequency when moving towards the cell, and decrease it when moving away from the cell, the increases/decreases being relative to the carrier frequency used in the network. The cell's antennas experience a similar Doppler shift with respect to the UE's uplink (transmitted) signals. The magnitude of the Doppler shift depends on the relative velocity of the UE towards the transmitting antenna. Deployment of cellular communication system antennas along a train track are particularly problematic because if the transmitting antennas are placed close to the track, the angle between the trajectory of the UE and the line between the UE and the transmitting antennas will be small. Consequently, a substantial part of the UE's velocity will contribute to a Doppler shift, and the higher the speed of the train, the more the Doppler shift. Moreover, there will be an abrupt change of sign of the Doppler shift when the UE passes the transmitting antenna, and the smaller the angle, the more abrupt the change is.
Given an angle, a, between a first vector representing the trajectory of the train (and hence also the UE) in Euclidean three-dimensional space and a second vector (also in Euclidean three-dimensional space) between the train and the transmitting antenna, the Doppler shift can be expressed as
      Δ    ⁢                  ⁢    f    =      f    ⁡          (                                                  1              -                              v                c                                                    1              +                              v                c                                                    -        1            )      where c is the speed of light and ν is the relative velocity of the UE (essentially the same as the velocity of the train) towards the transmitting antenna. With an angle α as discussed above and UE velocity, νUE, the relative velocity towards the transmitting antenna giving rise to Doppler shift is ν=νUE cos α.
To address this problem, a unidirectional beam arrangement has been proposed in, for example, 3GPP TSG-RAN WG4 Meeting #74bis, R4-151365 “Modified arrangement for RRH based model” (Rio de Janiro, Brazil, 20-24 Apr. 2015). Such an arrangement is also disclosed in PCT/EP2015/061759, filed May 27, 2015. With a unidirectional beam arrangement, the variability of the uplink (UL) Doppler shift is essentially zero along the entire track, except close to Remote Radio Head (RRH) sites (i.e., antenna sites), where it is not zero but is still reduced to a fraction of the maximum possible Doppler shift. Such an arrangement and its exemplary performance are illustrated by FIGS. 1A, 1B, 1C, 1D, and 1E.
In order to avoid UE 101 interruptions due to frequency retuning, cells 120, 122, 124 that form a super-cell use an antenna node 110, 112, 114 configuration in which a UE 101 always moves either towards or away from the antenna node 110, 112, 114 with which it communicates. This means the UE 101 will always experience either a positive or a negative Doppler shift, by which passing an antenna node 110, 112, 114 will only mean that a new path with essentially the same Doppler shift is becoming stronger (or weaker, depending on whether the UE is moving away from or towards the antenna node). In FIG. 1A, the example depicts the UE 101 moving westward, so it is moving towards each antenna node with which it communicates. The illustrated UE 101 would move away from each antenna node if it were moving eastward.
FIGS. 1B, 1C, 1D, and 1E depict an example of the Doppler shift and path loss experienced by a wireless communication device and a network node, when the wireless communication device (e.g., UE 101) is moving at a speed of 500 km/h with the communicating antenna node located ahead of the UE, with antenna node inter-site distance of 1000 m, and distance between site and path of 30 m, when using a carrier frequency of 2.7 GHz. More particularly:                FIG. 1A depicts an exemplary UE aboard a train travelling at 500 km/h between RRHs that are spaced 1000 m apart from one another, and located 30 m from the track.        FIG. 1B is a corresponding graph illustrating UE path loss as a function of the UE's position along the track.        FIG. 1C is a corresponding graph illustrating UE tracked frequency as a function of the UE's position along the track.        FIG. 1D is a corresponding graph illustrating RRH path loss as a function of the UE's position along the track.        FIG. 1E is a corresponding graph illustrating RRH received frequency as a function of the UE's position along the track.        
For the UE 101 moving towards the second antenna node 112, the path loss of the received downlink signal will gradually decrease as the distance between said device and the downlink transmit antenna(s) comprised in the second antenna node 112 decreases. The UE 101 passes the second antenna node 112 while beginning to receive from the first antenna node 110, and soon the UE 101 is no longer within the beam of the second antenna node 112. Consequently, the signal from the first antenna node 110 is experienced as stronger than the same signal from the second antenna node 112. Hence the wireless communication device will identify a new path, substantially at the same frequency offset as the previous path. The path loss between the first antenna node 110 and the UE 101 is initially at its maximum, but this rapidly reduces as the UE approaches the first antenna node 110.
The instantaneous frequency of the signal received by the wireless communication device (illustrated in FIG. 1C) may display a small spike or ripple when the UE is changing from one downlink beam to another. The magnitude of this momentary disturbance depends on at which angle (as measured between the wireless communication device and the antenna node) the signal transmitted from said antenna node becomes stronger than the signal from the previous antenna node. In the example, the momentary disturbance is about 350 Hz, to be compared with 2500 Hz had there been a switch in a conventional high-speed train scenario with bidirectional split beams.
The unidirectional beam arrangement provides a particular advantage in that it relies solely on network deployment configuration without the need for any accompanying adaptation of UE design.
One may assume that as many cells as possible share the same Physical Cell Identity (ID). There are two possibilities when it comes to RRH arrangement for uplink (UL) reception (RX) and downlink (DL) transmission (TX):                UL RX and DL TX lobes or beams oriented in the same direction (i.e., UL and DL beams both being oriented in a same direction along the track)        UL RX and DL TX lobes oriented in opposite directions (i.e., UL beams being oriented in one direction along the track and DL beams being oriented in an opposite direction along the track). This arrangement is illustrated in FIG. 2. Here, the UE receives DL transmissions from a first RRH (201) and transmits UL signals to an adjacent, second RRH (203).        
In deployments having UL RX and DL TX lobes oriented in opposite directions and operating as illustrated in FIG. 2, the received frequency in the base station is essentially the same as the transmitted frequency because the Doppler shifts in the DL and in the UL are the same but with opposite sign, and hence cancel each other out. This is because the UE sets its transmitted UL frequency based on the received DL frequency. Thus, if the UE moves westward (i.e., to the left in the figure), the received DL frequency at the UE will be shifted down from the nominal frequency. The UE sets its UL frequency based on this lowered DL frequency, and the Doppler shift to the RRH 203 is positive, so that the received UL frequency at the RRH 203 will be shifted back to the nominal frequency.
The inventors of the subject matter described herein have come up with a new RRH arrangement for a dedicated Single Frequency Network (SFN) High Speed Train (HST) scenario. See, for example, 3GPP contributions 3GPP TSG RAN WG4 Meeting #76, R4-154518, “TP Unidirectional RRH arrangement” (Aug. 24-28, 2015) and 3GPP TSG RAN WG4 Meeting #76bis, R4-155743, “Unidirectional RRH Arrangement for HST SFN” (Sophia Antipolis, France, Oct. 12-16, 2015) by the inventors. The arrangement is illustrated in FIG. 3, which shows that multiple RRHs (antenna nodes) are configured to form beams having a same orientation with respect to the track of the high-speed train, so that a unidirectional SFN is implemented. The UE receives primarily from one RRH at a time.
The arrangement of FIG. 3 has been shown to significantly improve throughput for a UE traveling at speeds up to at least 750 km/h, by stabilizing the Doppler shift experienced by the wireless communication device (e.g., UE) and thus the Doppler shift experienced by the network node (e.g. eNodeB) on the uplink. See, for example, 3GPP contributions 3GPP TSG RAN WG4 Meeting #76, R4-154520 “Evaluation of Unidirectional RRH arrangement for HST SFN” (Aug. 24-28, 2015) and 3GPP TSG RAN WG4 Meeting #76bis, R4-155758 “Priority of controlling interruptions” (Sophia Antipolis, France, Oct. 14-16, 2015). In addition to the stabilized frequency offsets experienced by wireless communication devices and network nodes, it has also been shown that this RRH arrangement results in negligible inter-carrier interference (ICI) which results in a higher signal to interference ratio (SIR), and low levels of fading, all together leading to a higher carrier to interference and noise ratio (CINR) than otherwise possible. This in turn allows higher modulation orders and less robust encoding to be used; that is, higher order modulation and coding schemes (MCS) can be used. Hence, the system throughput is improved. A thorough analysis can be found in 3GPP contribution 3GPP TSG RAN WG4 Meeting #76, R4-154516, “Modified RRH Arrangement for HST SFN” (Aug. 24-28, 2015).
It is noted that there is also a bi-static version in which UL RX and DL TX are oriented in opposite directions along the track.
FIG. 4 further illustrates a unidirectional RRH arrangement for a SFN network, this time specifically in a HST scenario. A first train 401 is travelling in one direction (e.g., Eastward) and a second train 403 is travelling in an opposite direction (e.g., Westward). Multiple users are onboard each respective train 401, 403. All UEs onboard each respective train experience and exhibit the same Doppler shift characteristics.
An aspect of the above-described arrangements involves extending each cell—as perceived by the UE—as far as possible to reduce the number of times a UE has to change cell (handover or reselection). The actual cell the UE sees while onboard a moving train may be a combined cell and/or several cells in a SFN that all share the same identity. Handovers of UEs are carried out between such cells, but it is handled on the network side without involving the UE.
At some point the UE may have to change cell. This may be because, for example, tracks that have been trunked together split up in different directions at some point, or it may be that the connection between the eNodeB and the RRHs reaches a limit with respect to latency and similar technical considerations.
Handing over to a new cell while travelling at 750 km/h is challenging, particularly if the handover involves an abrupt change in Doppler shift. A UE in general (legacy UE) cannot correctly determine a Doppler shift from one carrier to another that exceeds 2 kHz (so called capture range). This is because a wrap-around in the estimator results in the UE determining wrong frequency offset and consequently tuning its demodulation frequency towards an incorrect target. In many cases this results in radio link failure, by which the UE will have to go through a radio link re-establishment procedure. Radio link failures reduce the system capacity.
It is therefore desirable to provide technology that minimizes the number of occurrences of radio link failure when a UE onboard a high-speed train needs to perform a handover from one cell to another.